On Wed, Aug 26, 2015 at 10:55:06PM +0300, ville.syrjala@xxxxxxxxxxxxxxx wrote: > From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > > Calculate the number of retries we should do for each i2c-over-aux > message based on the time it takes to perform the i2c transfer vs. the > aux transfer. We assume the shortest possible length for the aux > transfer, and the longest possible (exluding clock stretching) for the > i2c transfer. > > The DP spec has some examples on how to calculate this, but we don't > calculate things quite the same way. The spec doesn't account for the > retry interval (assumes immediate retry on defer), and doesn't assume > the best/worst case behaviour as we do. > > Note that currently we assume 10 kHz speed for the i2c bus. Some real > world devices (eg. some Apple DP->VGA dongle) fails with less than 16 > retries. and that would correspond to something close to 15 kHz (with > our method of calculating things) But let's just go for 10 kHz to be > on the safe side. Ideally we should query/set the i2c bus speed via > DPCD but for now this should at leaast remove the regression from the > 1->16 byte trasnfer size change. And of course if the sink completes > the transfer quicker this shouldn't slow things down since we don't > change the interval between retries. > > I did a few experiments with a DP->DVI dongle I have that allows you > to change the i2c bus speed. Here are the results of me changing the > actual bus speed and the assumed bus speed and seeing when we start > to fail the operation: > > actual i2c khz assumed i2c khz max retries > 1 1 ok -> 2 fail 211 ok -> 106 fail > 5 8 ok -> 9 fail 27 ok -> 24 fail > 10 17 ok -> 18 fail 13 ok -> 12 fail > 100 210 ok -> 211 fail 2 ok -> 1 fail > > So based on that we have a fairly decent safety margin baked into > the formula to calculate the max number of retries. > > Fixes a regression with some DP dongles from: > commit 1d002fa720738bcd0bddb9178e9ea0773288e1dd > Author: Simon Farnsworth <simon.farnsworth@xxxxxxxxxxxx> > Date: Tue Feb 10 18:38:08 2015 +0000 > > drm/dp: Use large transactions for I2C over AUX > > v2: Use best case for AUX and worst case for i2c (Simon Farnsworth) > Add a define our AUX retry interval and account for it > > Cc: Simon Farnsworth <simon.farnsworth@xxxxxxxxxx> > Cc: moosotc@xxxxxxxxx > Tested-by: moosotc@xxxxxxxxx > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91451 > Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > --- > drivers/gpu/drm/drm_dp_helper.c | 81 ++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 79 insertions(+), 2 deletions(-) > > diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c > index 7069e54..23b9fcc 100644 > --- a/drivers/gpu/drm/drm_dp_helper.c > +++ b/drivers/gpu/drm/drm_dp_helper.c > @@ -424,6 +424,78 @@ static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter) > I2C_FUNC_10BIT_ADDR; > } > > +#define AUX_PRECHARGE_LEN 10 /* 10 to 16 */ > +#define AUX_SYNC_LEN (16 + 4) /* preamble + AUX_SYNC_END */ > +#define AUX_STOP_LEN 4 > +#define AUX_CMD_LEN 4 > +#define AUX_ADDRESS_LEN 20 > +#define AUX_REPLY_PAD_LEN 4 > +#define AUX_LENGTH_LEN 8 > + > +/* > + * Calculate the length of the AUX request/reply. Gives the "best" > + * case estimate, ie. successful while as short as possible. > + */ > +static int drm_dp_aux_req_len(const struct drm_dp_aux_msg *msg) > +{ > + int len = AUX_PRECHARGE_LEN + AUX_SYNC_LEN + AUX_STOP_LEN + > + AUX_CMD_LEN + AUX_ADDRESS_LEN + AUX_LENGTH_LEN; > + > + if ((msg->request & DP_AUX_I2C_READ) == 0) > + len += msg->size * 8; > + > + return len; > +} > + > +static int drm_dp_aux_reply_len(const struct drm_dp_aux_msg *msg) > +{ > + int len = AUX_PRECHARGE_LEN + AUX_SYNC_LEN + AUX_STOP_LEN + > + AUX_CMD_LEN + AUX_REPLY_PAD_LEN; > + > + /* > + * For read we expect what was asked. For writes there will > + * be 0 or 1 data bytes. Assume 0 for the "best" case. > + */ > + if (msg->request & DP_AUX_I2C_READ) > + len += msg->size * 8; > + > + return len; > +} > + > +#define I2C_START_LEN 1 > +#define I2C_STOP_LEN 1 > +#define I2C_ADDR_LEN 9 /* ADDRESS + R/W + ACK/NACK */ > +#define I2C_DATA_LEN 9 /* DATA + ACK/NACK */ > + > +/* > + * Calculate the length of the i2c transfer (in AUX clocks) > + * assuming the i2c bus speed is as specified. Gives the the > + * "worst" case estimate, ie. successful while as long as possible. > + * Doesn't account the the "MOT" bit, and instead assumes each > + * message includes a START, ADDRESS and STOP. Neither does it > + * account for additional random variables such as clock stretching. > + */ > +static int drm_dp_i2c_msg_len(const struct drm_dp_aux_msg *msg, > + int i2c_speed_khz) > +{ > + return (I2C_START_LEN + I2C_ADDR_LEN + msg->size * I2C_DATA_LEN + > + I2C_STOP_LEN) * 1000 / i2c_speed_khz; This doesn't seem to compute the lenght, but the time a transfer takes, in usec (if I haven't screwed up my numbers ...). Also DIV_ROUND_DOWN to make the defensiveness clear? > +} > + > +/* > + * Deterine how many retries should be attempted to successfully transfer Deteri_m_e. > + * the specified message, based on the estimated durations of the > + * i2c and AUX transfers. > + */ > +static int drm_dp_i2c_retry_count(const struct drm_dp_aux_msg *msg, > + int i2c_speed_khz) > +{ > + int aux_len = drm_dp_aux_req_len(msg) + drm_dp_aux_reply_len(msg); > + int i2c_len = drm_dp_i2c_msg_len(msg, i2c_speed_khz); > + > + return DIV_ROUND_UP(i2c_len, aux_len + AUX_RETRY_INTERVAL); So assuming I get things right i2c_len is actually i2c_time_us, but aux_len is still just a length (and since the bus runs at 2MHz it doesn't seem to just add up correctly). But AUX_RETRY_INTERVAL is in usec (and I expected an usec/usec division for the ration). I think there's a aux_time_us = DIV_ROUND_UP(aux_len * 1000 * 1000 / aux_speed_HZ) missing. Cheers, Daniel > +} > + > /* > * Transfer a single I2C-over-AUX message and handle various error conditions, > * retrying the transaction as appropriate. It is assumed that the > @@ -436,13 +508,18 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg) > { > unsigned int retry, defer_i2c; > int ret; > - > /* > * DP1.2 sections 2.7.7.1.5.6.1 and 2.7.7.1.6.6.1: A DP Source device > * is required to retry at least seven times upon receiving AUX_DEFER > * before giving up the AUX transaction. > + * > + * We also try to account for the i2c bus speed. > + * FIXME currently assumes 10 kHz as some real world devices seem > + * to require it. We should query/set the speed via DPCD if supported. > */ > - for (retry = 0, defer_i2c = 0; retry < (7 + defer_i2c); retry++) { > + int max_retries = max(7, drm_dp_i2c_retry_count(msg, 10)); > + > + for (retry = 0, defer_i2c = 0; retry < (max_retries + defer_i2c); retry++) { > mutex_lock(&aux->hw_mutex); > ret = aux->transfer(aux, msg); > mutex_unlock(&aux->hw_mutex); > -- > 2.4.6 > > _______________________________________________ > dri-devel mailing list > dri-devel@xxxxxxxxxxxxxxxxxxxxx > http://lists.freedesktop.org/mailman/listinfo/dri-devel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
